In many machine vision applications, changes in depth can provide much of the information for inspection, especially in cases where the grey scale or intensity contrast is poor, difficult to predict or irrelevant. In fact, it has been suggested that most industrial vision applications are inherently three-dimensional and that two-dimensional problems rarely exist. SMD (surface mounted device) inspection is a good example of an application where depth detection could be very useful for determining the presence and orientation of components. Otherwise, special case engineering is usually involved to handle varying component color, texture and background. Even with standard circuit inspection techniques, some capability for depth perception is desirable. For example, when operators inspect with a stereo microscope both color and depth perception capabilities are utilized.
Depth detection techniques are categorized as passive when a controlled source of radiation is not required, or active if a beam of radiant energy is involved. Passive ranging techniques avoid putting constraints on the observed objects or their environment and, consequently, have been the subject of much research in both computer vision and psychophysics. Methods based on stereo disparity, camera motion, surface reflectants, texture gradients, shadows and occlusions have been explored. These techniques often have psychophysical correlates. For example, depth perception in the human visual system is believed to be based upon these types of cues.
One disadvantage of the passive approach is the extensive computation required for construction of a depth map. Methods based on stereo disparity and camera motion are potentially very powerful but require matching of corresponding features in a sequence of images. A method for consistently establishing the correspondence has not been developed at this time for real time computer applications. Nevertheless, several ideas have emerged from studies in depth perception, including techniques for representing the properties of surfaces.
Active depth detection techniques eliminate the correspondence problem and measure the depth directly by using a beam of energy and recording the time of flight (sonar and radar applications such as shown in U.S. Pat. No. 4,212,534). Depth may also be measured through displacement (triangulation and grid coding), phase shift of a laser beam compared to a reference beam (laser radar), or shadow length (directional illumination). Extensive computation for a depth map is avoided and the information processing task is reduced to extraction of three-dimensional features, representation of the surfaces and scene analysis operations. In the application where the use of intensity or color improve classification, both range and intensity data may be used.
The triangulation or structured light concept offers a great potential for acquiring a dense, high-resolution (approximately 1 mil and finer) 3-D image at high data rates (10 MHz) at a relatively low cost. The triangulation concept is one of the oldest depth detection techniques which exists, but which continues to undergo new developments. On the other hand, the laser radar approach is relatively new to machine vision. While the laser radar approach has some advantages, its relatively low data rate and high cost make this approach somewhat unwieldy for high resolution application; as the modulation frequency is increased to the GHz range, high resolution imaging becomes relatively difficult to implement in a cost-effective way. By contrast, the triangulation method is relatively simple and has an inherently high resolution capability.
Many of the refinements of the basic triangulation concept involve projection of single and multiple stripes (grid patterns), scanning strip systems and flying spot scanners. One 3-D vision system utilizing structured light is described in U.S. Pat. No. 4,105,925. The vision system described therein includes a linear array sensor which is positioned so that a line of light is visible only if a reference plane is illuminated. If an object is present, then the light beam is broken. A second line source is used to minimize shadows. As the object is scanned with a linear sensor, a binary image is produced and the presence and orientation of the object is then determined.
One commercially available 3-D vision system which produces height measurements includes a microprocessor-based, laser line sectioning system. The system is capable of producing 60 fields of 480 x,y,z coordinates each second which corresponds to approximately 30 KHz data rate. Each line of data requires acquisition of an entire video field. If a one-half inch by one-half inch object is to be imaged at one mil, x,y,z resolution then the maximum speed of the object conveyed on a conveyor belt for 100% inspection of the part is approximately 30 seconds per inch.
Such a single stripe system is most useful in gauging and spot checking and the use of multiple stripes in such a system are best for highly constrained scenes which are not likely to change very often. These systems are mostly used for gauging rather than for image processing and 100% inspection. Proper sampling and acquisition of dense three-dimensional information requires the stripes be scanned across the object and imaged with an array camera or line sensor, either of which can limit the data rate. Tradeoffs between speed, resolution and dynamic range are a necessary consequence of the use of a multiple stripe system.
One method for acquiring data in a 3-D vision system is to replace the line scan or array sensor utilized in most 3-D vision systems with a lateral effect photodiode as illustrated in U.S. Pat. No. 4,375,921. The ambiguities which might exist in multiple stripe systems are not a problem with this technique and the measurement range variation is relatively large. This is true because the entire detector surface is available and there is no requirement to share the detector area as in a multiple stripe system.
Unfortunately, the bandwidth of most of the devices with amplification circuitry is well below one MHz. Dual detector devices (i.e. bi-cells) which have a 30 MHz bandwidth are available but standard devices are not useful in the basic triangulation concept for imaging under low light conditions at high speed, particularly when large fields of view are examined. These devices are also very sensitive to spot shape and geometric distortions.
U.S. Pat. Nos. 4,068,955 and 4,192,612 disclose a thickness measuring device utilizing well-known trigonometric principles to generate data to give the distance to or the thickness of a remote object. In such thickness gauging systems, beams of light are directed through beam-splitting mirrors to opposite surfaces of the object to be measured. By ascertaining the relative angles of incidence and reflection with respect to the object surface, suitable trigonometric rules can be applied to generate the approximate thickness of the object in question.
U.S. Pat. No. 4,472,056 discloses a shape-detecting apparatus for detecting three-dimensional products or parts such as soldered areas of a printed circuit board, the parts attached to the printed board and bumps in an LSI bonding process. The apparatus comprises a slit projector for projecting a slit bright line image forming lens for forming the bright line image. The apparatus also comprises an image scanning mechanism for the bright line image formed through an image forming lens in a height direction of the object and a one-dimensional image sensing device for self-scanning the bright line image formed therein with an array of image sensing elements orthogonal to the scanning direction by the image scanning mechanism. This system is severely limited by readout time; each 3-D point requires examination at many photodetectors.
U.S. Pat. No. 4,355,904 discloses a device for measuring depth using a pair of photodetectors such as photodiodes and a partially reflective and a partially transmissive filter. The computation of the centroid is done by an analog divider.
U.S. Pat. No. 4,553,844 discloses a method and system in which a spot beam scans an object in one direction and the resulting spot image is detected through observation in a direction transverse the one direction.
U.S. Pat. No. 4,645,917 discloses a swept aperture flying spot profiler. The sensor used in the system is either a photomultiplier or an avalanche diode.
U.S. Pat. No. 4,349,277 discloses a parallax method of wavelength labeling based on optical triangulation. A signal processor calculates a normalized signal that is independent of surface reflectivity and roughness variations.
U.S. Pat. No. 4,634,879 discloses the use of optical triangulation for determining the profile of a surface utilizing two photomultiplier tubes in a flying spot camera system. These are arranged in a "bi-cell" configuration. As an anti-noise feature, amplitude modulation is impressed upon the laser beam and a filter network is used to filter photomultiplier response so as to exclude response to background optical noise.
Other United States patents of a more general interest include U.S. Pat. Nos. 4,053,234; 4,065,201; 4,160,599; 4,201,475; 4,249,244; 4,269,515; 4,411,528; 4,525,858; 4,567,347; and 4,569,078.